1. Field of the Invention
The present invention relates to a method of producing a carbon-doped amorphous silicon thin film to be used as a material of a light incident layer (i.e., window layer) of an amorphous solar battery.
2. Description of the Related Art
In an amorphous solar battery, it is necessary to reduce the optical absorption coefficient of the window layer so as to allow as much incident light as possible to reach the layer of the solar battery having a photoelectric conversion function. In order to reduce the absorption coefficient of the window layer, it is necessary that the window layer have a large optical band gap. It is sufficient that the window layer of an amorphous silicon solar battery be comprised of a carbon-doped amorphous silicon thin film having an optical band gap larger than that of the materials comprising the solar battery itself. Conventionally, such a carbon-doped amorphous silicon thin film has been produced in the following manner.
First, a gaseous mixture of a mono-silane gas (SiH.sub.4) and an acetylene gas (C.sub.2 H.sub.2) is led into a vacuum chamber so as to make the pressure in the chamber 10-1000 Pa Next, a 13.56 MHz high frequency electric field is applied across a pair of parallel-plate electrodes disposed in the vacuum chamber so that a glow discharge is generated across the electrodes. The gaseous mixture is decomposed by energy from the glow discharge, and a carbon-doped amorphous silicon thin film is deposited on a substrate which has been placed in the vacuum chamber and heated to 150.degree.-300.degree. C. The carbon concentration x in the film is a function of the ratio of the acetylene flow rate F(C.sub.2 H.sub.2) to the mono-silane flow rate F(SiH.sub.4), and is represented by equation (1) based on data presented by Asano et al. in Journal cf Applied Physics, Vol. 63 (1989), No. 7, p. 2346. EQU x=0.68[F (C.sub.2 H.sub.2)/F(SiH.sub.4)].sup.0.466 ( 1)
Further, the magnitude of the optical band gap Eg (unit: electron volt) of the film is a linear function of the carbon concentration x in the film, and is represented by equation (2). EQU Eg=1.72+1.54x (2)
Therefore, the optical band gap of the formed carbon-doped amorphous silicon thin film can be successively altered by varying the mixing ratio of monosilane to acetylene in the raw material gas.
In the conventional method for producing such a carbon-doped amorphous silicon thin film, however, there are disadvantages in that the defect density of the resultant carbon-doped amorphous silicon thin film sharply increases as the carbon concentration in the carbon-doped amorphous silicon thin film increases. This leads to a deterioration of the film's photoelectric characteristics such as electric conductivity (photoconductivity) or the like. The photoconductivity of a carbon-doped amorphous silicon thin film having an optical band gap, for example, of 2.0 electron volts is low, about 10.sup.-8 s/cm while under irradiation of an artificial sun having intensity of 100 mW/cm.sup.2.